This invention relates generally to the controlled chemical reduction of surface films, and more specifically, to a process for removing a native film from a metal surface so that another metal can be joined to that surface.
In the process of joining two metallized objects, such as by soldering, welding, or the like, a film of metallic oxide on the surface of one of the metallized objects can impede the process of joining. The metal oxide can totally prevent the joining, make the joining unreliable, or make the use of flux or other material or process necessary to effect an acceptable joining. For example, when soldering to a nickel surface, a nickel oxide layer provides such an impediment. Because nickel oxidizes very readily and rapidly upon exposure to air, soldering to nickel is difficult because of the almost omnipresent oxide layer. Soldering to nickel is, therefore, usually accomplished by using a soldering flux, by high temperature reduction of the oxide in a hydrogen ambient, or by other techniques for reducing the oxide layer.
In the fabrication of semiconductor devices, for example, it is often necessary to join a metallized semiconductor die to a metallized package or to join an electrical lead to a metallized region on the surface of the semiconductor device. The joining can be by soldering or by one of the bonding techniques such as ultrasonic or thermo-compression bonding. In any of these joining techniques the presence of an oxide layer on the metal surface impedes the joining. The very nature of semiconductor devices, as well as the nature of many other structures, limits the use of standard techniques for reducing the oxide prior to joining. For example, many semiconductor devices involve very shallow device regions which would be adversely affected by high temperature reduction techniques. Large area devices such as photovoltaic cells are too large and to fragile to permit the use of solder fluxes since the size of the device makes the proper cleanup of the device difficult is not impossible. Additionally, the use of flux with any device entails additional processing steps, including cleanup, which significantly add to the cost of the device. Maintaining a controlled reducing atmosphere is expensive, requiring large quantities of gases as well as equipment.
A need therefore existed for a process which would overcome the deficiencies of the prior art processes to allow the joining of metallic parts.
It is therefore an object of this invention to provide an improved process for applying an electrical contact to a semiconductor device.
It is another object of this invention to provide an improved process for fabricating a photovoltaic cell.
It is yet another object of this invention to provide an improved process for bonding together metal objects.
It is a further object of this invention to provide an improved process for removing an oxide layer or other native film layer from a metallized surface prior to soldering.